 # BNDSPANTREE - Editorial

Authors: Ihor Barenblat, Matvii Aslandukov
Developer: Ihor Barenblat
Editorialist: Ihor Barenblat

# DIFFICULTY:

3221

We see that for this subtask there is no mst rule’’. So we just need to find an assignment of the edge weights using integers from the range [1, m]. It is possible to do this using greedy approach: set 1 to the edge that have l_i = 1 with the minimum possible r_i. This is true, since any valid assignment can be changed without losing validity to be compatible with the mentioned greedy assignment. So the overall greedy algorithm is the following: for each i from 1 to m set i as edge weight for the edge with l_j \le i with the minimum possible r_j, between all edges with unset weight. Time complexity: O(m \cdot log(m)).

Lets call the non-spanning-tree edge as special’’. After the assigning a value for the special edge we need to be sure that there exist possibility of assigning for spanning-tree edges weights from the range [1, m] (except the already assigned one). Lets find all suitable possible values that satisfy this condition, choose the greatest one from the range of the special edge and update r_j for the edges on the cycle to be sure that they are smaller that the value we assigned to the special edge. For finding all suitable possible values that satisfy mentioned condition we can use Hall’s marriage theorem on the bipartite graph (first part consists of vertices corresponding to the edges and second part consists of vertices corresponding to possible edge weights): value k is suitable if and only if there is no L,R such that L \le k \le R and R-L+1 \le (# i such that 1 \le i \le (n-1) and L \le l_i \le r_i \le R). Time complexity: O(m \cdot log(m)).

Lets call pair of edges (e_{tree}, e_{non\_tree}) interesting (here e_{tree} is a spanning-tree edge and e_{non\_tree} is a non-spanning-tree edge) if e_{tree} lies on the simple path in the expected minimum spanning tree between vertices that connects e_{non\_tree}. It is easy to see that for each interesting pair of edges we can do the following update: e_{tree}.r = min(e_{tree}.r, e_{non\_tree}.r - 1), e_{non\_tree}.l = max(e_{non\_tree}.l, e_{tree}.l + 1). With such modifications, any assignment found by the greedy algorithm from the Subtask 1 will satisfy mst rule’’. Time complexity: O(m \cdot n).